1. Field of the Invention
The present invention relates to an anti-skid control system for controlling braking force applied to road wheels in the braking operation of an automotive vehicle to prevent the road wheels from being locked, and more particularly to an anti-skid control system for providing a low-select simultaneous control mode for rear road wheels of the vehicle.
2. Description of the Prior Art
In order to prevent road wheels from being locked, there has been employed an anti-skid control system which controls the braking force by decreasing, increasing, or holding a hydraulic braking pressure supplied to each wheel brake cylinder operatively connected to each road wheel. According to the anti-skid control system, in general, a rotational speed of each road wheel or a wheel speed is detected, in response to which the hydraulic braking pressure in each wheel brake cylinder is controlled so as to obtain the maximum coefficient of friction. With respect to the rear road wheels of the vehicle, the braking force is applied simultaneously to both the rear road wheels, and a so-called low-select simultaneous control mode is provided for the rear road wheels. Namely, according to the low-select simultaneous control mode, a braking force, which is to be applied to one of the rear road wheels rotating at the lower speed and having a tendency to be locked first, is applied to both of the right and left rear road wheels simultaneously. In the anti-skid control system, as disclosed in the U.S. Pat. No. 5,015,041 for example, an independent control mode or the low-select simultaneous control mode is selected for a pair of road wheels such as the rear road wheels, depending upon a vehicle speed or a coefficient of friction of a road surface, to ensure a running stability and shorten a stopping distance.
In the case where the vehicle is turning, a lateral load shift occurs on the rear road wheels due to a centrifugal force. Therefore, a large load is applied to the rear road wheel at the outer side, and a load applied to the rear road wheel at the inner side is reduced, so that the inner side wheel has a large tendency to be locked. Consequently, in the case where a simultaneous control mode is set on the basis of the road wheel rotating at the lower speed, such as the low-select simultaneous control mode set for the rear road wheels, the braking force which should be applied to the road wheel at the inner side is reduced by a force shifted to the road wheel at the outer side due to the load shift.